2.3
Plant Immunity Enhanced by Trichoderma Species and Their
Fungal Metabolites
Trichoderma can induce an enhanced defensive capacity in plants that provide
protection against a broad spectrum of plant pests [5, 7, 34]. Fungal molecules that
trigger plant defense responses are known as elicitors. In general, glycoproteins,
carbohydrates, fatty acids, peptides, and extracellular microbial enzymes are nonspecific elicitors [12, 53].
The signal molecules involved in the establishment of plant defense responses
elicited by Trichoderma are just beginning to be identified [7, 12, 16]. Several
Trichoderma species produce trichothecenes, most notably trichodermin and
harzianum A (HA) [54, 55]. Recently, Malmierca and coworkers [56] reported that
disruption of the gene tri4 of Trichoderma arundinaceum IBT 40837 (Ta37), which
encodes a cytochrome P450 monooxygenase that oxygenates trichodiene to give rise
to isotrichodiol, reduced the antifungal activity against B. cinerea and R. solani and
the ability to induce the expression of SA- and JA-responsive genes in comparison
with the wild-type strain, indicating that HA plays an important function in the
sensitization of Ta37-pretreated plants against pathogens. Furthermore, trichodiene
is able to elicit the expression of Botrytis genes involved in the synthesis of botrydial
and also induces the terpene gene expression in Trichoderma strains [57].
Djonovic and coworkers [58] analyzed the pattern of proteins secreted by
T. virens strain Gv29-8. Electrophoretic analysis of protein extracts revealed
a remarkable abundance of a low molecular weight protein. The protein was
designated as Sm1 (small protein). The amino acid composition of Sm1 revealed
a high percentage of hydrophobic residues (40%), including four cysteines and three
tryptophans, and its characteristics were consistent with those reported to fungal
elicitors. The exogenous application of the proteinaceous elicitor Sm1 of T. virens in
cotton (Gossypium hirsutum) roots induced the expression of defense-related genes
such as GLU (β-1,3-glucanase), CHT (chitinase), POD6 (peroxidase), and GhLOX1
(lipoxygenase1). Furthermore, 0.5 nmol of Sm1 in cotton cotyledons was able to
induce resistance against the foliar pathogen Colletotrichum sp. Sm1 also triggered
defense responses in maize plants [59]. Similarly, T. atroviride secretes a Sm1homologous protein, Epl1, in the presence of maize roots which is released as
dimer, but in the monomeric form triggers effective defense responses against the
pathogenic fungus Colletotrichum graminicola [60]. Trichoderma formosa also
produces a small peptide elicitor of plant defense homologous similar to the
cerato-platanin Epl1. Epl1 from T. formosa is a 12 kDa peptide [61]. Ruocco and
coworkers [62] reported that T. longibrachiatum MK1 and other fungal strains
produce a hydrophobin type II that has 71 amino acids and a molecular weight of
7218 Da, which is able to activate plant defense and enhance root branching in
tomato seedlings.
Different Trichoderma strains also produce non-ribosomal peptides (NRPs) that
activate plant defense responses and have antibiotic properties against different types
of fungi [12, 30]. NRPs result from fusion of at least two amino acids by multimodular mega-enzymes, called non-ribosomal peptide synthetases (NRPSs) outside
272
H. A. Contreras-Cornejo et al.
Plant Immunity Enhanced by Trichoderma Species and Their
Fungal Metabolites
Trichoderma can induce an enhanced defensive capacity in plants that provide
protection against a broad spectrum of plant pests [5, 7, 34]. Fungal molecules that
trigger plant defense responses are known as elicitors. In general, glycoproteins,
carbohydrates, fatty acids, peptides, and extracellular microbial enzymes are nonspecific elicitors [12, 53].
The signal molecules involved in the establishment of plant defense responses
elicited by Trichoderma are just beginning to be identified [7, 12, 16]. Several
Trichoderma species produce trichothecenes, most notably trichodermin and
harzianum A (HA) [54, 55]. Recently, Malmierca and coworkers [56] reported that
disruption of the gene tri4 of Trichoderma arundinaceum IBT 40837 (Ta37), which
encodes a cytochrome P450 monooxygenase that oxygenates trichodiene to give rise
to isotrichodiol, reduced the antifungal activity against B. cinerea and R. solani and
the ability to induce the expression of SA- and JA-responsive genes in comparison
with the wild-type strain, indicating that HA plays an important function in the
sensitization of Ta37-pretreated plants against pathogens. Furthermore, trichodiene
is able to elicit the expression of Botrytis genes involved in the synthesis of botrydial
and also induces the terpene gene expression in Trichoderma strains [57].
Djonovic and coworkers [58] analyzed the pattern of proteins secreted by
T. virens strain Gv29-8. Electrophoretic analysis of protein extracts revealed
a remarkable abundance of a low molecular weight protein. The protein was
designated as Sm1 (small protein). The amino acid composition of Sm1 revealed
a high percentage of hydrophobic residues (40%), including four cysteines and three
tryptophans, and its characteristics were consistent with those reported to fungal
elicitors. The exogenous application of the proteinaceous elicitor Sm1 of T. virens in
cotton (Gossypium hirsutum) roots induced the expression of defense-related genes
such as GLU (β-1,3-glucanase), CHT (chitinase), POD6 (peroxidase), and GhLOX1
(lipoxygenase1). Furthermore, 0.5 nmol of Sm1 in cotton cotyledons was able to
induce resistance against the foliar pathogen Colletotrichum sp. Sm1 also triggered
defense responses in maize plants [59]. Similarly, T. atroviride secretes a Sm1homologous protein, Epl1, in the presence of maize roots which is released as
dimer, but in the monomeric form triggers effective defense responses against the
pathogenic fungus Colletotrichum graminicola [60]. Trichoderma formosa also
produces a small peptide elicitor of plant defense homologous similar to the
cerato-platanin Epl1. Epl1 from T. formosa is a 12 kDa peptide [61]. Ruocco and
coworkers [62] reported that T. longibrachiatum MK1 and other fungal strains
produce a hydrophobin type II that has 71 amino acids and a molecular weight of
7218 Da, which is able to activate plant defense and enhance root branching in
tomato seedlings.
Different Trichoderma strains also produce non-ribosomal peptides (NRPs) that
activate plant defense responses and have antibiotic properties against different types
of fungi [12, 30]. NRPs result from fusion of at least two amino acids by multimodular mega-enzymes, called non-ribosomal peptide synthetases (NRPSs) outside
272
H. A. Contreras-Cornejo et al.